riscv: doc: Add relative doc to describe RISC-V

Add documents to describe NX25 and AE250.
Also update other documents for RISC-V.

Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Rick Chen <rickchen36@gmail.com>
Signed-off-by: Greentime Hu <green.hu@gmail.com>

README

@@ -143,6 +143,7 @@ Directory Hierarchy:
   /nios2		Files generic to Altera NIOS2 architecture
   /openrisc		Files generic to OpenRISC architecture
   /powerpc		Files generic to PowerPC architecture
+  /riscv		Files generic to RISC-V architecture
   /sandbox		Files generic to HW-independent "sandbox"
   /sh			Files generic to SH architecture
   /x86			Files generic to x86 architecture
@@ -3510,7 +3511,7 @@ Low Level (hardware related) configuration options:
 		globally (CONFIG_CMD_MEMORY).
 
 - CONFIG_SKIP_LOWLEVEL_INIT
-		[ARM, NDS32, MIPS only] If this variable is defined, then certain
+		[ARM, NDS32, MIPS, RISC-V only] If this variable is defined, then certain
 		low level initializations (like setting up the memory
 		controller) are omitted and/or U-Boot does not
 		relocate itself into RAM.
@@ -4964,6 +4965,22 @@ On NDS32, the following registers are used:
 NOTE: DECLARE_GLOBAL_DATA_PTR must be used with file-global scope,
 or current versions of GCC may "optimize" the code too much.
 
+On RISC-V, the following registers are used:
+
+	x0: hard-wired zero (zero)
+	x1: return address (ra)
+	x2:	stack pointer (sp)
+	x3:	global pointer (gp)
+	x4:	thread pointer (tp)
+	x5:	link register (t0)
+	x8:	frame pointer (fp)
+	x10-x11:	arguments/return values (a0-1)
+	x12-x17:	arguments (a2-7)
+	x28-31:	 temporaries (t3-6)
+	pc:	program counter (pc)
+
+    ==> U-Boot will use gp to hold a pointer to the global data
+
 Memory Management:
 ------------------
 

doc/README.NX25

@@ -0,0 +1,46 @@
+NX25 is Andes CPU IP to adopt RISC-V architecture.
+
+Features
+========
+
+CPU Core
+ - 5-stage in-order execution pipeline
+ - Hardware Multiplier
+	 - radix-2/radix-4/radix-16/radix-256/fast
+ - Hardware Divider
+ - Optional branch prediction
+ - Machine mode and optional user mode
+ - Optional performance monitoring
+
+ISA
+ - RV64I base integer instructions
+ - RVC for 16-bit compressed instructions
+ - RVM for multiplication and division instructions
+
+Memory subsystem
+ - I & D local memory
+   - Size: 4KB to 16MB
+ - Memory subsyetem soft-error protection
+   - Protection scheme: parity-checking or error-checking-and-correction (ECC)
+   - Automatic hardware error correction
+
+Bus
+ - Interface Protocol
+   - Synchronous AHB (32-bit/64-bit data-width), or
+   - Synchronous AXI4 (64-bit data-width)
+
+Power management
+ - Wait for interrupt (WFI) mode
+
+Debug
+ - Configurable number of breakpoints: 2/4/8
+ - External Debug Module
+   - AHB slave port
+ - External JTAG debug transport module
+
+Platform Level Interrupt Controller (PLIC)
+ - AHB slave port
+ - Configurable number of interrupts: 1-1023
+ - Configurable number of interrupt priorities: 3/7/15/63/127/255
+ - Configurable number of targets:  1-16
+ - Preempted interrupt priority stack

doc/README.ae250

@@ -0,0 +1,137 @@
+Andes Technology SoC AE250
+===========================
+
+AE250 is the mainline SoC produced by Andes Technology using NX25 CPU core
+base on RISC-V architecture.
+
+AE250 has integrated both AHB and APB bus and many periphals for application
+and product development.
+
+NX25-AE250
+=========
+
+NX25-AE250 is the SoC with AE250 hardcore CPU.
+
+Configurations
+==============
+
+CONFIG_SKIP_LOWLEVEL_INIT:
+	If you want to boot this system from SPI ROM and bypass e-bios (the
+	other boot loader on ROM). You should undefine CONFIG_SKIP_LOWLEVEL_INIT
+	in "include/configs/nx25-ae250.h".
+
+Build and boot steps
+====================
+
+build:
+1. Prepare the toolchains and make sure the $PATH to toolchains is correct.
+2. Use `make nx25-ae250_defconfig` in u-boot root to build the image.
+
+Verification
+====================
+
+Target
+====================
+1. startup
+2. relocation
+3. timer driver
+4. uart driver
+5. mac driver
+6. mmc driver
+7. spi driver
+
+Steps
+====================
+1. Define CONFIG_SKIP_LOWLEVEL_INIT to build u-boot which is loaded via gdb from ram.
+2. Undefine CONFIG_SKIP_LOWLEVEL_INIT to build u-boot which is booted from spi rom.
+3. Ping a server by mac driver
+4. Scan sd card and copy u-boot image which is booted from flash to ram by sd driver.
+5. Burn this u-boot image to spi rom by spi driver
+6. Re-boot u-boot from spi flash with power off and power on.
+
+Messages
+====================
+U-Boot 2018.01-rc2-00033-g824f89a (Dec 21 2017 - 16:51:26 +0800)
+
+DRAM:  1 GiB
+MMC:   mmc@f0e00000: 0
+SF: Detected mx25u1635e with page size 256 Bytes, erase size 4 KiB, total 2 MiB
+In:    serial@f0300000
+Out:   serial@f0300000
+Err:   serial@f0300000
+Net:
+Warning: mac@e0100000 (eth0) using random MAC address - be:dd:d7:e4:e8:10
+eth0: mac@e0100000
+
+RISC-V # version
+U-Boot 2018.01-rc2-00033-gb265b91-dirty (Dec 22 2017 - 13:54:21 +0800)
+
+riscv32-unknown-linux-gnu-gcc (GCC) 7.2.0
+GNU ld (GNU Binutils) 2.29
+
+RISC-V # setenv ipaddr 10.0.4.200 ;
+RISC-V # setenv serverip 10.0.4.97 ;
+RISC-V # ping 10.0.4.97 ;
+Using mac@e0100000 device
+host 10.0.4.97 is alive
+
+RISC-V # mmc rescan
+RISC-V # fatls mmc 0:1
+   318907   u-boot-ae250-64.bin
+     1252   hello_world_ae250_32.bin
+   328787   u-boot-ae250-32.bin
+
+3 file(s), 0 dir(s)
+
+RISC-V # sf probe 0:0 50000000 0
+SF: Detected mx25u1635e with page size 256 Bytes, erase size 4 KiB, total 2 MiB
+
+RISC-V # sf test 0x100000 0x1000
+SPI flash test:
+0 erase: 36 ticks, 111 KiB/s 0.888 Mbps
+1 check: 29 ticks, 137 KiB/s 1.096 Mbps
+2 write: 40 ticks, 100 KiB/s 0.800 Mbps
+3 read: 20 ticks, 200 KiB/s 1.600 Mbps
+Test passed
+0 erase: 36 ticks, 111 KiB/s 0.888 Mbps
+1 check: 29 ticks, 137 KiB/s 1.096 Mbps
+2 write: 40 ticks, 100 KiB/s 0.800 Mbps
+3 read: 20 ticks, 200 KiB/s 1.600 Mbps
+
+RISC-V # fatload mmc 0:1 0x600000 u-boot-ae250-32.bin
+reading u-boot-ae250-32.bin
+328787 bytes read in 324 ms (990.2 KiB/s)
+
+RISC-V # sf erase 0x0 0x51000
+SF: 331776 bytes @ 0x0 Erased: OK
+
+RISC-V # sf write 0x600000 0x0 0x50453
+device 0 offset 0x0, size 0x50453
+SF: 328787 bytes @ 0x0 Written: OK
+
+RISC-V # crc32 0x600000 0x50453
+crc32 for 00600000 ... 00650452 ==> 692dc44a
+
+RISC-V # crc32 0x80000000 0x50453
+crc32 for 80000000 ... 80050452 ==> 692dc44a
+RISC-V #
+
+*** power-off and power-on, this U-Boot is booted from spi flash 	***
+
+U-Boot 2018.01-rc2-00032-gf67dd47-dirty (Dec 21 2017 - 13:56:03 +0800)
+
+DRAM:  1 GiB
+MMC:   mmc@f0e00000: 0
+SF: Detected mx25u1635e with page size 256 Bytes, erase size 4 KiB, total 2 MiB
+In:    serial@f0300000
+Out:   serial@f0300000
+Err:   serial@f0300000
+Net:
+Warning: mac@e0100000 (eth0) using random MAC address - ee:4c:58:29:32:f5
+eth0: mac@e0100000
+RISC-V #
+
+TODO
+====================
+
+Boot bbl and riscv-linux

doc/README.standalone

@@ -58,6 +58,7 @@ Design Notes on Exporting U-Boot Functions to Standalone Applications:
 	Blackfin	0x00001000	0x00001000
 	NDS32		0x00300000	0x00300000
 	Nios II		0x02000000	0x02000000
+	RISC-V		0x00600000	0x00600000
 
    For example, the "hello world" application may be loaded and
    executed on a PowerPC board with the following commands: